- #1
kgal
- 41
- 0
How many watts should a 40w bulb use at 120v?
Is a 40w Bulb Efficient at 120v?
Click For Summary
Discussion Overview
The discussion centers around the efficiency and power consumption of a 40-watt light bulb when used at a voltage of 120 volts. Participants explore the relationship between wattage, voltage, and the actual power usage of light bulbs, including historical and regional variations in bulb ratings.
Discussion Character
- Technical explanation
- Debate/contested
Main Points Raised
- Some participants assert that a 40-watt bulb is designed to use 40 watts at the specified voltage of 120 volts.
- Others mention that the actual power consumption may vary due to various factors, suggesting that the rated wattage does not always reflect the true usage.
- One participant highlights the historical context of bulb ratings in different regions, noting that UK bulbs are rated at 240 volts, which affects their expected performance and longevity.
- There is a suggestion that the wattage rating does not indicate the amount of light produced, which raises questions about the efficiency of the bulb.
Areas of Agreement / Disagreement
Participants generally agree that a 40-watt bulb is rated for that wattage at 120 volts, but there is disagreement regarding the implications of this rating on actual power consumption and light output. The discussion remains unresolved regarding the efficiency of the bulb and the factors affecting its performance.
Contextual Notes
Participants mention that electrical supply can vary, which may influence the actual power consumption of bulbs. There are also references to historical practices in bulb rating that may not align with current standards.
- #2
jedishrfu
Mentor
- 15,681
- 10,475
40 watts?
wikipedia has a discussion on watts and watt-hours that might clear up things.
wikipedia has a discussion on watts and watt-hours that might clear up things.
- #3
AlephZero
Science Advisor
Homework Helper
- 6,999
- 299
THe bulb would be marked as using 40W at a specified voltage. If you don't know that information, you can't answer the question.
- #4
mathman
Science Advisor
Homework Helper
- 8,130
- 575
In the U.S. standard voltage is 120 volts, so light bulbs will use the wattage as marked.
- #5
cmb
- 1,128
- 128
Due to what I believe is partly an historic anomaly, bulbs in the UK are rated at 240V, but 230V is the EU voltage standard which should be delivered to UK homes.
I think in part it is usual to rate the bulb at a higher voltage as the life expectancy [of filament bulbs] drops exponentially once above the rated voltage. So bulbs may be sold into a market with voltage ratings slightly in excess of the specified volts.
Electrical supply into homes is rarely that accurate, at least in UK. My own house suffered for years at variable under-voltage. We share a relatively small spur with an industrial estate. It dropped (I kid you not) to 183V on one occasion I was measuring it myself. I complained to the elec co and they did some work. I then had >250V for several months until I complained enough they installed £70,000 of new kit in the local sub-station.
I think in part it is usual to rate the bulb at a higher voltage as the life expectancy [of filament bulbs] drops exponentially once above the rated voltage. So bulbs may be sold into a market with voltage ratings slightly in excess of the specified volts.
Electrical supply into homes is rarely that accurate, at least in UK. My own house suffered for years at variable under-voltage. We share a relatively small spur with an industrial estate. It dropped (I kid you not) to 183V on one occasion I was measuring it myself. I complained to the elec co and they did some work. I then had >250V for several months until I complained enough they installed £70,000 of new kit in the local sub-station.
- #6
Naty1
- 5,605
- 40
That's a power use figure, like one on a frig or work tool...it's not an indicator
of how much light it produces...
of how much light it produces...
- #7
cmb
- 1,128
- 128
Naty1 said:
Hence the question "How many watts should a 40w bulb use..."
- #8
pantaz
- 588
- 2
As previously stated, forty watts at the voltage specified by the manufacturer.cmb said:Hence the question "How many watts should a 40w bulb use..."
Due to a variety of factors, the measured power consumption will vary from the advertised value.
- #9
cmb
- 1,128
- 128
pantaz said:
Yes, I know that, but Naty 1 didn't appear to get the question.
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